As a young woman, Mercédès is kind of one-dimensional character. She's beautiful, exotic, and faithful. She has a kind heart. She's happy for Edmond when he returns and she cries for him when he's taken away. She tolerates Fernand, despite his threatening demeanor, and lets him know that she's willing to sacrifice her life if he should try to hurt Edmond. What more could you ask for in a woman? Well, maybe a little more character development.
But then we meet older Mercédès, Fernand's wife. What's the deal with her? How could she have abandoned Edmond? He was only gone for like, fourteen years right? What an ungrateful…wait, what's that? FOURTEEN YEARS? OK, sure, she married a big jerk, but on the plus side she raised a pretty cool, if cocky, son.
She's one of the first characters to recognize Monte Cristo as Edmond; she accepts her fate and leaves quickly after Fernand is exposed, and she's suitably chastened by the experience. So, looking at all that you'd think she might deserve a break, right? Heck, she even has to send her only son away to war. Mercédès ends up being her own harshest critic: "You did have faith," she tells Edmond, "you had strength, you trusted in God, and God sustained you. I was a coward, I denied Him, so God abandoned me; and here I am!" (112.105).
This is the last we hear of Mercédès. The Count moves on, finds love with Haydée, and rides off into the sunset. Meanwhile, Mercédès left to live in the old Dantès apartment in Marseille, left to cry herself to sleep, just as she was left to cry when Edmond was taken away to prison. Unwilling to "hope" and "wait" – the two things the Count tells Valentine and Maximillian they must do – she suffers as a result. But, really, hasn't she suffered enough? Is this punishment really called for?